Use of polyalcohols as polymer stabilizers

ABSTRACT

Natural cyclic polyalcohols such as polyfructoses and, dehydration products of sugar alcohols can be used for stabilising thermoplastic polymers, at a ratio of 0.001-5 phr of polyol with respect to the polymer. The cyclic polyol is in particular inulin or sorbitan. The polymer to be stabilised is in particular PVC, PE, PP or a halogenated rubber.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the national stage under 35 U.S.C. 371 ofPCT/NL97/00709, filed Dec. 18, 1997.

The present invention relates to the use of polyalcohols for stabilisingpolymers, in particular vinyl chloride polymer and to polymersstabilised with polyalcohols.

The manufacture of plastic articles from organic thermoplastic polymersrequires high temperatures (160° C. and higher), which leads to partialdegradation of the polymer and hence to lower mechanical performance anddiscoloring of the product. The problem is especially serious for vinylhalide polymers such as PVC. Stabilisers and co-stabilisers are commonlyused for preventing such thermo-oxidative degradation of polymers. Suchstabilisers also protect finished polymer-based articles againstdegradation processes resulting from the action of heat, oxygen and/or(UV) light. Suitable stabilisers should not only prevent degradation ofthe polymer, but must also be compatible with the various othercompounds of the polymer blend, avoiding the release of volatile orother components as a result of dehomogenising, which components maylimit the utility of the polymer or may be detrimental to health.

Until now the so-called primary PVC stabilisers have often been based onheavy metal containing compounds, such as cadmium, barium, tin and leadcompounds. These primary stabilisers are capable of irreversibly bindinghydrogen chloride. More recently inorganic stabilisers based on e.gcalcium, zinc, aluminium and magnesium layered structures have beendeveloped. However, their performance is still insufficient to fullyreplace the heavy metal stabilisers. Further stabilisation is achievedby means of metal-free secondary stabilisers such as epoxy compounds,organic phosphites, antioxidants and light stabilisers. It has beenknown that hydrogen chloride, but also certain metal chlorides such aszinc chloride, catalyst the degradation of PVC. This unfavourable effectof metal chlorides may be reduced by a reaction with metal compoundssuch as calcium stearate or e.g. by complexing with polyalcohols and/ororganic phosphites or β-diketo compounds. Polyalcohols, such assorbitol, mannitol, lactitol, (di)pentaerythritol andtris(2-hydroxyethyl) isocyanurate (THEIC) have been proposed as(co)stabilisers for organic polymers.

Various patents describe the use of polyalcohols as stabilises for PVC.Examples of these are DE-A-2728865 which describes the use of mannitol,sorbitol or xylitol, together with calcium and zinc stearates andstearoylbenzoylmethane for stabilising PVC. WO 93/07208 discloses a PVCstabiliser system formed from zinc oxide and penta-erythritol. The useof maltitol and lactitol as stabilisers is described in EP-A-677549.Long-chain fatty acid partial esters of polyols such as polyglucose orsorbitol have been proposed as PVC stabilisers in DE-A-3536936 andDE-A-3643968, respectively. SU patent 863602 teaches the use of xylitanfor improving the thermal and mechanical stability of butadiene/styrenelatex and similar latices.

Known stabilisers such as sorbitol, mannitol and xylitol showdisadvantages in that, although they give a good heat stability, theyhave a negative effect on the discolouring of the polymer duringprocessing. The more effective ones, such as THEIC, are ratherexpensive.

It has been found now that thermoplastic polymers, as vinyl chloridepolymers, can be effectively stabilised by the addition of certainnatural cyclic polyalcohols. These cyclic polyalcohols increase theheat-stability of the polymers and at the same time do not substantiallycontribute to discolouration. The cyclic polyalcohols are especiallynon-toxic, food-compatible carbohydrates. Suitable carbohydrates includenon-reducing oligo- and poly-saccharides, di- and oligo-saccharides thereducing unit of which has been reduced, acid-catalysed dehydrationproducts of sugar alcohols. Non-reducing polysaccharides preferably havechain lengths of less than 100 monosaccharide units. Examples of cyclicpolyalcohols are polyfructoses such as inulin, and levan, and the cyclicmono-dehydration products of sugar alcohols including xylitol,arabinitol, sorbitol (glucitol), galactitol (dulcitol), mannitol,iditol, and higher analogues. The mono-dehydrated products of thehexitols are typically 2-(1,2-dihydroxyethyl)-3,4-dihydroxy-oxolanes or(less commonly) the isomeric2,5-bis(hydroxymethyl)-3,4-dihydroxy-oxolanes or2-hydroxymethyl-3,4,5-trihydroxy-oxanes and of the pentitols they areusually 2-hydroxymethyl-3,4-dihydroxy-oxolanes. The anhydrohexitols arepreferred. The mono-dehydration products of xylitol, sorbitol, andgalactitol are also known as xylitan, sorbitan and galactitan,respectively. Where reference is made hereafter to anhydro-polyols orsorbitan, these terms also include the anhydro-derivatives of the othersugar alcohols, especially galactitan. Mixtures of anhydropolyols canalso be used advantageously, as such components often have complementingstabilising effects.

Preferably at last one of the components is an anhydrohexitol. Examplesof useful mixtures are sorbitan/xylitan sorbitan/anhydrolactitol,mannitan/galactitan, sorbitan/inulin, and in particular a mixture ofsorbitan and galactitan, e.g. in ratios between 1:3 and 3:1.

The cyclic, non-reducing carbohydrates such as inulin and sorbitan canbe used in polymer compounds in a manner known per se. The stabiliserscan be mixed with other additives, such as impact modifiers for rigidformulations (for example chlorinated polyethylene orbutadiene/styrene/(acrylonitrile) co or ter-polymers), plasticisers forflexible formulations (for example phthalic esters such as dibutylphthalate or dioctyl phthalate, aliphatic monobasic or dibasic esterssuch as butyl oleate, epoxidised soybean oil, dioctyl adipate), fillers,pigments, flow modifiers (for example acrylates), lubricants (forexample calcium stearate, zinc stearate, fatty esters and amides), flameretardants (for example aluminium hydroxide, antimony trioxide),phosphites (for example triaryl phosphites or aryl-alkyl phosphites),antioxidants (for example hindered phenols), HALS (hindered amine lightstabiliser) compounds, UV absorbers (for example benzophenones such as2-hydroxy-4-methoxybenzophenone, benzotriazoles, salicylates), ketoesters and ketones such as N-phenyl-3-acetyl-2,4-pyrrolidine-2,4-dione;other stabilisers such as β-diketones and β-keto esters,β-aminocrotonates including dihydropyridine-3, 5-dicarboxylic esters,uracils, other polyol co-stabilisers such as pentaerythritol,tris-(hydroxyethyl isocyanurate), mannitol and the like may also be usedat reduced levels. Examples of suitable formulations are given ascompounds A, B and C below. The formulations are then processed into ashaped article by means of calendering, rotational moulding, spreadcoating, slush moulding, extrusion, injection moulding, blow-moulding orother conventional technique.

Preferentially, the polyol stabilisers are used in combination with acalcium salt such as calcium stearate and/or zinc compounds, such aszinc stearate or zinc oxide. Inulin and the anhydropolyols arepreferably used at a level of 0.001-5%, especially 0.01-2%, mostpreferably 0.05-1% with respect to the thermoplastic polymer. Anotherclass of (co)stabilisers to be used advantageously in combination withthe present polyols are the anionic clays, such as alkali metalalumosilicates and other zeolyte-type compounds, and layered multimetalsalts commonly referred to as hydrotalcites. The hydrotalcites areconsidered as an anionic clay with an overall chemical composition of:M²⁺ _(x)M³⁺ _(y)(OH)_(2x+3y−2)CO₃ , in which M²⁺ is a bivalent cationlike Mg, Zn, Ni etc. and M³⁺ is a trivalent cation, in particular Al.The carbonate group can be exchanged by other anions or anioniccomplexes such as hydroxide, nitrate, sulphate, iodide, bromide,chloride, fluoride, oxalate and other (di)carboxylates, oxide,perchlorate and silicate. Typical examples are Al₂Mg₆(OH)₁₆CO₃.4H₂O andAl₂Mg₄(OH)₁₂CO₃.3H₂O. The weight ratios between polyol and calcium saltor zinc compound or hydrotalcites or other anionic clay is generallybetween 10:1 and 1:100, more in particular between 3:1 and 1:10. Thesestabiliser combinations as such are also part of the invention.

The thermoplastic polymer can be e.g. polyethene, polypropene,polystyrene, halogenated rubber, fluorine-containing polymers, such aspoly(vinylidene fluoride), poly(vinylidene-chloride) and, especially,poly(vinyl chloride). PVC, the non-vinyls (polyethene and polypropene)and halogenated rubbers and other halogen-containing polymers such asPVDC, as well as copolymers with other monomers and mixtures with otherpolymers are preferred according to the invention.

The heat stability of a polymer like PVC can be expressed in the heatingtime at a selected temperature (e.g. 200° C.) until the polymer degradesas determined by the colour turning brown to black. The test can e.g. beperformed in a Matthis oven using a 25 cm polymer strip which isstepwise moved from the oven.

Non-vinyl polymers as PE are investigated in multi-extrusion tests,after the first, third and fifth run, the colour properties and the meltflow index are measured. Polyolefins are very sensitive to UV-light,therefore UV-stabilising tests are carried out.

The colour properties can be expressed as the whiteness according toBerger (Wb (%)) and the Yellowness index (Yi (%)). Both can bedetermined e.g. using a Minolta Chromameter with a DP 301 dataprocessor. The rating is done according to the CIE L-a-b system (CIE:Commission International d'Eclairage). White/black (L), green/red (a)and yellow/blue (b) values are converted to the Wb and Yi values. Foroptimum performance, the heat stability and the whiteness should be ashigh as possible, and the yellowness index should be as low as possible.

The favourable properties of sorbitan are illustrated in tables 1 and 2.Sorbitol and sorbitan were incorporated in polymer compounding mixturesA and B respectively given below (pbr=parts per 100 with respect topolymer). it can be ween from these tables that sorbitol increases heatstability, but at the same time drastically deteriorates the colourproperties when compared with the blanc. Depending on the particularcompound, sorbitan has a somewhat lower (A) or somewhat higher (B)stabilizing effect but the colour properties remain unaffected. Whereasthe lower stabilising effect of sorbitan, if at all present, can becompensated by using other additives, the poor colour properties ofsorbitol cannot be compensated easily by using other components.

The favorable properties of inulin art illustrated in tables 3 and 4.Inulin (fractionated chicory inulin, average DP 21.4) was incorporatedat different levels in polymer compounding mixture A and B respectively.While the heat stability is increased substantially, both the whitenessand the yellowness index remain virtually unchanged. Table 5 shows theeffect of average degree of polymerisation (DP) of the inulin on thestabilising properties. The preferred average DP is between 5 and 40, inparticular between 10 and 25. Preferably, the inulin is substantiallyfree from mono- and di-saccharides. The favourable effect ofanhydrohexitols and hydrotalcite on the heat stability is shown by table6.

Compound A (Rigid PVC): Polyvinylchloride S-PVC (K-68) 100 parts Impactmodifier acrylate compounds    2-15 phr Filler chalk    2-12 phr Pigmenttitanium dioxide    1-10 phr Flow modifier acrylate ester homopolymer 0.1-3 phr Metal stabiliser calcium/zinc stabiliser  0.1-5 phr LubricantPE wax 0.01-2 phr Compound B (Rigid PVC): Compound A + Polyol THEIC  0-1 phr Polyol lactone 0.01-2 phr Polyol partial ester 0.01-1 phrLayered clay anionic clay 0.01-2 phr Costabiliser β-diketone 0.01-2 phrOrganic phosphite organic diphosphite 0.01-2 phr Compound C (FlexiblePVC): Polyvinylchloride S-PVC (K-71) 100 parts Plasticiser dioctylphthalate   10-50 phr Epoxy plasticiser epoxidised soybean oil  0.3-10phr Lubricant PE wax 0.01-2 phr Organic zinc salt liquid zinc stabiliser0.01-2 phr Inorganic stabiliser anionic clay 0.01-5 phr

TABLE 1 Heat stability of rigid PVC (compound A) stabilised by sorbitolor sorbitan heat-stability (minutes) Wb (%) Yi (%) blanc 21.4 46.5 15.1sorbitol 0.5 phr 34.9 16.5 26.3 sorbitan 0.5 phr 27.0 42.0 16.8

TABLE 2 Heat stability of rigid PVC (compound B) stabilised by sorbitolor sorbitan heat-stability (minutes) Wb (%) Yi (%) blanc 58.5 62.2 9.6sorbitol 0.5 phr 70.2 37.6 18.8 sorbitan 0.5 phr 75.0 62.9 9.5

TABLE 3 Heat stability of PVC stabilised with inulin (compound A)heat-stability (minutes) Wb (%) Yi (%) blanc 21.4 46.5 15.1 inulin 0.1phr 23.4 43.7 15.9 inulin 0.5 phr 27.0 37.1 17.7 inulin 1.0 phr 29.727.7 20.7

TABLE 4 Heat stability of PVC stabilised with inulin (compound B)heat-stability (minutes) Wb (%) Yi (%) blanc 58.5 62.2 9.6 inulin 0.1phr 67.5 61.1 9.9 inulin 0.5 phr 69.8 56.7 11.0 inulin 1.0 phr 72.0 52.512.1

TABLE 5 Influence of inulin chain length on stability and colourproperties DP Inulin heat stability (0.5 phr) (minutes) Wb (%) Yi (%) -(blanc) 57.6 62.3 9.5 3.4 65.3 44.7 14.3 6.6 68.9 45.2 13.8 10.3 71.652.0 12.4 13.3 72.0 54.5 11.6 21.4 72.0 54.1 11.8 25.0 70.7 53.3 11.9

TABLE 6 Heat stability of flexible PVC (compound C) stabilised withhydrotalcite (Alc 4) and sorbitan/galactitan stabiliser (phr)heat-stability Alc 4 sorbitan galactitan (minutes) Wb (%) Yi (%) none(blanc) 24.8 42.0 13.4 0.15 — — 29.0 31.7 17.8 — 0.3 — 27.0 40.8 14.00.15 0.15 — 32.4 28.4 19.8 — 0.15 0.15 27.9 33.2 15.9 0.15 0.075 0.07542.3 20.9 22.2

What is claimed is:
 1. A process of stabilizing a thermoplastic polymercomprising incorporating in said thermoplastic polymer an effectiveamount for stabilization of at least one cyclic polyol selected from thegroup consisting of polyfructoses having an average chain length of 3-60anhydrofructose units and dehydration products of hexitols.
 2. Theprocess according to claim 1, wherein 0.001-5 phr of polyol isincorporated with respect to the polymer.
 3. The process according toclaim 2, wherein 0.01-5 phr of polyol is incorporated with respect tothe polymer.
 4. A process for stabilizing a thermoplastic polymercomprising incorporating in said thermoplastic polymer an effectiveamount for stabilization of inulin having an average chain length of3-60 anhydrofructose units.
 5. The process according to claim 4, whereinthe inulin contains less than 1% of glucose and fructose.
 6. A processof stabilizing a thermoplastic polymer comprising incorporating in saidthermoplastic polymer an effective amount for stabilization of at leastone of sorbitan and galactitan.
 7. The process according to claim 1,wherein the thermoplastic polymer is selected from the group consistingof halogen-containing polymers, polyethylene and polypropylene.
 8. Aprocess of stabilizing a thermoplastic polymer comprising incorporatingin said thermoplastic polymer an effective amount for stabilization ofat least one cyclic polyol selected from the group consisting ofpolyfructoses and dehydration products of hexitols, wherein the cyclicpolyol is combined with an additive which is an organic calcium salt, azinc compound, an anionic clay, or a mixture of two or more of saidadditives.
 9. A stabilizing composition comprising one or more cyclicpolyols selected from the group consisting of poly-fructoses anddehydration products of sugar alcohols, combined with one or moreinorganic anionic clay stabilizers.
 10. The stabilizing compositionaccording to claim 9, comprising at least one anhydro-hexitol and ahydrotalcite.
 11. The stabilizing composition according to claim 9,wherein the weight ratio of cyclic polyol to inorganic stabilizer isbetween 10:1 and 1:100.
 12. In a compounded mixture of a thermoplasticpolymer and a stabilizer, the improvement wherein said stabilizercomprises 0.01-2 phr, with respect to the polymer, of a cyclic polyolselected from the group consisting of inulin having an average chainlength of 3-60 anhydrofructose units, sorbitan, galactitan and a mixtureof at least two of said cyclic polyols.
 13. The compounded mixtureaccording to claim 12, further containing 0.01-2 phr of at least one ofan organic calcium salt, a zinc compound and an anionic clay.
 14. Ashaped article comprising thermoplastic polymer stabilized byincorporation therein of 0.01-2 phr, with respect to the polymer, of atleast one cyclic polyol selected from the group consisting ofpolyfructoses having an average chain length of 3-60 anhydrofructoseunits and dehydration products of hexitols.
 15. The process according toclaim 4, wherein the inulin has an average chain length of 5-40anhydrofructose units.
 16. The process according to claim 15, whereinthe inulin has an average chain length of 10-25 anhydrofructose units.17. The process according to claim 4, wherein the inulin contains lessthan 1% of glucose, fructose and sucrose.
 18. The process according toclaim 1, wherein the thermoplastic polymer is selected from the groupconsisting of vinyl chloride polymers and copolymers.
 19. Thestabilizing composition according to claim 9, wherein the weight ratioof cyclic polyol to inorganic stabilizer is between 3:1 and 1:10.